Ferroelectric memories have received much attention as dense, randomly addressable, non-volatile memories. A ferroelectric memory element is essentially a capacitor consisting of two electrodes and ferroelectric material filling the gap between the electrodes. At a minimum, the ferroelectric material has a very high dielectric constant, thus providing an improved dynamic random-access memory (DRAM) capacitor. Furthermore, the electrodes can induce two stable polarization states in the ferroelectric dependent on the polarity of the applied voltage, and the ferroelectric remains in the selected state after the voltage has been removed from the electrode. Thus, a ferroelectric memory can be non-volatile.
Early ferroelectric memories relied on relatively thick ferroelectric bodies on opposing side of which electrodes were formed, for example, by deposition. Such a configuration is incompatible with large-scale integration and requires operational voltages considerably higher than those used with semiconductor integrated circuits. In later ferroelectric memories, a ferroelectric thin film was deposited on a metallic bottom electrode, and a top electrode was deposited on the ferroelectric thin film. This configuration and fabrication process, similar to those in semiconductor memories, offer much promise, but the polycrystalline structure of the ferroelectric film resulting from the metal substrate introduces many problems.
Recently, Ramesh et al. have disclosed singly crystalline, epitaxially grown ferroelectric memories in U.S. patent application, Ser. No. 07/616,166, filed Nov. 10, 1990, in "Epitaxial Cuprate Superconductor/Ferroelectric Heterostructures," Science, volume 252, May 17, 1991, pp. 944-946, and in "Ferroelectric PbZr.sub.0.2 Ti.sub.0.8 O.sub.3 thin films on epitaxial Y-Ba-Cu-O," Applied Physics Letters, volume 59, Dec. 30, 1991, pp. 3542-3544. The patent application is incorporated herein by reference. The bottom electrode of these memories is a singly crystalline thin film of a high-temperature perovskite superconductor, for example, YBa.sub.2 Cu.sub.3 O.sub.7-x (hereinafter YBCO). The ferroelectric thin film is then epitaxially deposited on the YBCO film, and, although it is cubic or nearly cubic, its crystalline orientation epitaxially follows the underlying perovskite crystalline orientation.
Although YBCO is a superconductor at temperatures of less than 92.degree. K., the ferroelectric memories described above are intended to operate at room temperature where the YBCO exhibits high but normal conductivity, rather than the semiconductive or dielectric behavior observed in most non-superconducting perovskites.
The ferroelectric memories of Ramesh et al., although impressive, suffered from many problems. Their hysteresis curves were insufficiently square, with low coercive fields. Such memory elements would require relatively complicated transistorized gates to be written and read. Furthermore, these singly crystalline ferroelectric memory cells aged and fatigued rather rapidly under simulated usage. A typical requirement for volatile semiconductor memories is that they survive 10.sup.12 to 10.sup.15 read/write cycles. Such a high number is probably not required for non-volatile ferroelectric memories, but a minimum life of 10.sup.8 cycles would be desirable to assure system performance.